Last year, Google stopped to support Gmail offline with Google Gears, which was working well on Internet Explorer, Firefox and Chrome, and started working on an alternative.
It finally works, after more than 6 months where we have been left without a good solution. But it works only on Chrome browser.
It now supports up to 31 days of email and downloads all attachments. I know many that will appreciate.
Announcement is there :
You have to install this extension
By the way, Google Docs also has some offline capabilities with the Google Docs extension.
It still does not allow offline editing, but I bet they are working hard on this one.